Repeated Phenotypic Evolution by Different Genetic Routes in Pseudomonas fluorescens SBW25

Area of Science:

• Microbial evolution
• Genetics and genomics
• Bacterial adaptation

Background:

• Repeated evolution of similar phenotypes is common across life.
• Understanding conserved genetic mechanisms is crucial.
• Colony switching in Pseudomonas fluorescens SBW25 was previously studied in one lineage.

Purpose of the Study:

• To investigate the genetic basis of colony switching in a second independent lineage (Line 6) of Pseudomonas fluorescens SBW25.
• To compare the genetic mechanisms of colony switching between different lineages.
• To test a proposed molecular model for capsule expression regulation.

Main Methods:

• Genetic analysis of nine mutations in Line 6.
• Identification of the key mutation responsible for colony switching.
• Comparison of genetic mechanisms with Line 1.
• Assessment of gene expression changes related to capsule production.

Main Results:

• Colony switching in Line 6 is caused by a single nonsynonymous point mutation in the housekeeping sigma factor gene, rpoD.
• This rpoD mutation leads to increased ribosomal gene expression.
• The observed genetic and phenotypic changes in Line 6 are consistent with a model of capsule expression regulation involving intracellular regulators.

Conclusions:

• Colony switching can evolve through distinct genetic routes (rpoD vs. carB mutations) in independent lineages.
• The rpoD mutation in Line 6 drives colony switching by increasing colanic acid-like polymer (CAP) expression.
• The findings support a conserved regulatory model for capsule expression, despite different initiating mutations.